The essential reason is that the main parties are run predominantly by liberals who are anti-worker and addicted to cheap foreign labour.
The Conservative Party does have anti-migrant elements, but they are not generally in charge of policy.
It is difficult to disentangle Brexit and Covid, as they both occured at roughly the same time, but there was a significant loss of (particularly temporary) migrant labour due to Covid/Brexit.
For a short while, full employment ensued (but without any explicit Keynesian policy or intention), with wages rising keenly in low-wage occupations where EU migrants had come to predominate (but were now excluded by default), and trade union activity soaring.
To douse all of this and return the economy to attack mode against workers, the floodgates were reopened on a massive scale again and work visas handed out liberally.